1. Filed of the Invention
This invention relates to capacitance-type displacement measuring device, which is suitable for small measuring device such as electric micrometer, inner diameter measuring machine and angle gauge.
2. Description of Related Art
Conventionally, "cylindrical displacement sensor", which is shown in European Patent Application Publication No. EP 0 716 290 A2, is known as an electronic measuring machine which detects displacement amount. This machine will be explained below with reference to FIG. 10 and 11.
FIG. 10(A) is a perspective disassembled view of a half-cylindrical body 111 as a stator and a cylindrical body 112 as a rotor, FIG. 10(B) is a side view thereof, and FIG. 1(C)(D) are each unrolled view of the half-cylindrical body 111 and the cylindrical body 112. The half-cylindrical body 111 is fixed, and the cylindrical body 112 can be rotated around the same axis as the half-cylindrical body 111 at a predetermined gap (0.1 to 0.2 mm).
As shown in FIG. 10(C), a transmitting electrode group 113, and receiving electrodes 114 which are insulated from the transmitting electrode group 113, are each formed on inner circumferential face of the half-cylindrical body 111. In this example, transmitting electrode group 113 is located over predetermined length from an end of the half-cylindrical body 111, extending narrow in axial direction, and at the same pitch in parallel to the axis, with eight electrodes as one unit. And alternating currents which have each different phase by 45 degrees are impressed to each of the electrode of this transmitting electrode group 113. Receiving electrode 114 is located on the other end of inner circumferential face of the half-cylindrical body 111 in half-ring-belt-shape along circumferential direction.
The transmitting electrode group 113 and receiving electrode 114 are pattern-formed on a FPC substrate 117, which is shown in a dotted line in this example. On the FPC substrate, along with the electrode patterns, lead-lines 115, 116 of the electrode patterns are formed, and they are attached to the inner circumferential face of half-cylindrical body 111. The lead-lines are drawn out from both ends of the half-cylindrical body 111.
As shown in FIG. 10(D), coupling electrode 118 and ground electrode 119 are each formed on an outer circumferential face of the cylindrical body 112. Two coupling electrodes 118 are formed at the same pitch as the width of eight electrodes of the transmitting electrode group 113 in circumferential direction to face four electrodes of the transmitting electrode group 113 on the half-cylindrical body 111, and at the same time, to face the receiving electrode 114 so that capacitive coupling occurs between these electrodes. This coupling electrode 118 is provided to receive a signal caused by capacitive coupling between the transmitting electrode group 113 and the coupling electrode 118 and to transfer the signal to receiving electrode 114 by capacitive coupling. These electrodes are formed on the FPC substrate of the outer circumferential face of the cylindrical body 112. Incidentally, the ground electrode 119 can be omitted.
In this structure, as cylindrical body 112 is rotated in a direction shown by an arrow in FIG. 10(A), an amount of displacement of the cylindrical body 112 relative to the half-cylindrical body 111 can be measured by detecting a phase of receive signal produced by the capacitive coupling between the transmitting electrode group 113 and the coupling electrode 118 resulting from the rotation displacement. In this case, as the transmitting electrode group 113 are divided into eight, and each electrode is actuated at a different-phase by 45 degrees, the rotation displacement amount can be measured at an accuracy at which the pitch of the coupling electrode 118 are divided into eight.
FIG. 11 briefly shows general structure of measuring circuit 120 which actuates the displacement sensor shown in FIG. 10 and processes signals. This measuring circuit 120 is composed of an oscillator 121 which outputs predetermined clock pulses, and a pulse modulator 112 which impresses eight-phased alternating-signals on each electrode of the transmitting electrodes group 113 each having different phases by 45 degrees. The output signal from the receiving electrode 114 is inputted to a phase comparator 124 via an integrator 123. The phase comparator 124 compares the phase of this input signal with a standard phase and detect the amount of rotation displacement as phase lag of input signal relative to a standard phase, and inputs the detected signal to a counter 125. The counter 125 counts the clock pulse outputted from the oscillator 121, and digitally displays the amount of rotation displacement on an indicator 126.
To achieve sufficient measuring accuracy in the aforementioned cylindrical displacement sensor, concentric accuracy between the half-cylindrical body 111 as a stator and the cylindrical body 112 as a rotor is needed. Specifically, a gap between the stator and the rotor need to be retained fixed in circumferential direction. But in producing a small-type cylindrical displacement sensor, it is very difficult to achieve highly accurate concentricity between the stator and the rotor.
Highly accurate measuring can be achieved even with low concentricity between the stator and the rotor by locating two units of electrodes in the circumferential direction and averaging the outputs of these units, thus offsetting the effect of low concentricity. But if it is needed to locate two units of electrodes in the circumferential direction to avoid problems occurred by low concentricity, production efficiency lowered because more wiring is needed between the sensors and outside electric circuits.
It is an object of this invention to solve the conventional problem and to provide capacitance-type displacement measuring device which can keep high concentricity between the stator and the rotor in a simple structure.